Multi-reference algebraic diagrammatic construction theory for excited states: General formulation and first-order...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The algebraic diagrammatic construction (ADC) theory is a powerful method for calculating electronic states.
• Strong electron correlation in ground or excited states poses challenges for conventional methods.
• Accurate computation of spectroscopic properties requires methods that include all relevant orbitals.

Purpose of the Study:

• To generalize the algebraic diagrammatic construction (ADC) theory to a multi-reference framework (MR-ADC).
• To develop an efficient and reliable computational approach for systems with strong electron correlation.
• To enable the computation of spectroscopic properties, including transition amplitudes and spectral densities, for complex electronic systems.

Main Methods:

• Generalization of the effective Liouvillian formalism to multi-determinant reference states.
• Perturbative analysis of the multi-reference ADC (MR-ADC) formulation.
• Implementation of the first-order MR-ADC approximation (MR-ADC(1)).

Main Results:

• The MR-ADC approach is applicable to systems with strong electron correlation.
• MR-ADC accounts for electronic transitions involving core, active, and external orbitals.
• MR-ADC(1) successfully calculates excitation energies for the Be atom, LiF avoided crossing, and C2 doubly excited states.

Conclusions:

• The developed MR-ADC theory provides a robust framework for studying electron correlation.
• MR-ADC offers efficient computation of spectroscopic properties beyond conventional perturbation theories.
• The MR-ADC(1) implementation serves as a foundation for a hierarchy of advanced computational methods.